1. Field of the Invention
The present invention relates to a magnetic circuit for a speaker, especially a cone speaker for use in various audio equipments.
2. Description of the Related Art
It is well known that in a cone speaker, a current strain, which is attributable to a change in the position of a voice coil relative to a gap incurred by a large amplitude of bass, or attributable to a magnetic material forming a magnetic circuit, is conventionally addressed, for example, such that a short-circuiting ring made of copper or aluminum is put inside a pole piece or magnet and shorted thereby reducing an inductance of the voice coil closer to zero.
A short-circuiting ring disclosed in Japanese Patent No. 2737273 is made of a copper plate, formed into a cylinder, and attached to a center pole so as to be concentric with and parallel to a voice coil. The short-circuiting ring is adapted to short an eddy current flowing in the center pole with a voice current flowing in the voice coil thereby increasing the sound pressure level in a high frequency region.
Another short-circuiting ring is disclosed in Japanese Utility Model Publication No. Sho 63-29358, which has a slit formed in a longitudinal direction and is attached to a center pole, and still another short-circuiting ring is disclosed in Japanese Utility Model No. 2586414, which is composed of a meshwork comprising a plurality of electrically conductive thin wires meshed in a cylindrical form so as to be extendable, and which is attached to the side of the center pole except a gap portion.
In the above conventional measures, in which the short-circuiting ring as the secondary winding of the voice coil is shorted thereby reducing the inductance of the voice coil, and is disposed outside the center pole or inside the magnet thereby holding down a second harmonic distortion, the gap may possibly be expanded thereby reducing the magnetic flux density, and also the short-circuiting ring attached near the magnet has little influence on an impedance in a high-frequency region thus not promising much effect.
Under the above circumstances, a speaker is disclosed in Japanese Patent Publication No. Hei 7-32514, in which no short-circuiting ring is used, and a magnet for a magnetic circuit is formed of an electrically conductive ferrite containing divalent iron. This, however, invites an increase in magnet cost, raising problems with practical use.